1. Field of the Invention
The present invention relates to a PLL (Phase Locked Loop) speed control circuit which serves to control two or more objects based on a predetermined speed ratio.
2. Description of the Related Art
As an example, an electronic duplicating machine employing two or more document platforms requires the circuit for controlling two or more objects at a predetermined speed. This type of electronic duplicating machine is required to keep constant an optical distance from a document surface to a photosensitive drum irrespective of the scanning position. It, of course, includes a first mirror platform mounting a light source for radiating light on a document and a second mirror platform mounting a light mirror and a lens for guiding the light reflected from the document surface to the photosensitive drum. The former platform is adapted to be driven in the document-scanning direction at a speed twice as fast as the latter one. Traditionally, the travelling speeds of both platforms have been mechanically arranged to be a ratio of 2 to 1. That is, a wire-pulley mechanism has been used for the arrangement so that a rotary motor connected to the wire pulley is controlled to drive the first and the second mirror platforms at a ratio of 2 to 1. This wire-pulley mechanism disadvantageously cannot adapt to a high-speed-driven electronic duplicating machine, because the mechanism entails an elastic material, that is, a wire included in a transmission system for the driving force. In order to overcome this disadvantage, the newly-developed electronic duplicating machine has employed two linear motors This type of duplicating machine includes two PLL speed control circuits. These circuits serve to control the two linear motors so that these motors can stably drive the first mirror platform at a speed twice as fast as the second mirror platform, respectively.
The foregoing art is, however, designed to allow two PLL speed control circuits to respectively control the linear motors. It brings about the following disadvantage. That is, though the art is able to properly control the speeds of the linear motors, it does not offer an automatic modification function of any slippage caused about the positional relation between the first and the second mirror platforms, that is, an optical distance between the document surface and the photosensitive drum, which slippage results in shifting a duplicated image out of focusing. This is because these linear motors are individually arranged in speed and phase so that they disallow those platforms to return to their proper tracking positions even if they are shifted out of their locking ranges. It will be easily understood that this adverse effect increases more as time passes.
The electronic duplicating machine employing two linear motors is, in essence, suitable for a high-speed-driven machine, but has the foregoing great disadvantage which becomes an obstacle of upgrading the performance of the electronic duplicating machine.